This invention relates to a dielectric filter for use in various kinds of radiocommunication equipment.
FIG. 3(a) is a perspective view of the construction of a conventional dielectric filter. FIG. 3(b) is a cross-sectional view taken along the line A--A' of FIG. 3(a). FIG. 3(c) is a cross-sectional view taken along the line B--B' of FIG. 3(a). Referring to FIGS. 3(a) to 3(c), a generally box-like dielectric body 1 has an open end 1A formed as its one end face, and through holes 2a, 2b, and 2c formed so as to extend from the open end 1A to a short-circuit end 1B provided at the other end face. The length of the through holes 2a, 2b, and 2c is set to approximately 1/4 the wavelength .lambda. corresponding to the desired resonance frequency. Each of the through holes 2a, 2b, and 2c has a circular cross-sectional shape and the diameter of each hole is changed at an intermediate position from a smaller diameter on the open end side to a larger diameter on the short-circuit end side. Inner conductors 3a, 3b, and 3c are formed on inner surfaces of the through holes 2a, 2b, and 2c, respectively. An outer conductor 4 is formed over outer surfaces of the dielectric block 1 except for the open end 1A. The inner conductors 3a, 3b, and 3c and the outer conductor 4 are connected at the short-circuit end 1B of the dielectric body 1. The inner conductors 3a and 3c are connected to input/output terminals Ta and Tc through input/output capacitors Ca and Cc, respectively.
In this type of coupling distribution constant line circuit open at its one end and connected at the other end, coupling in an odd mode is strong at the open end 1A while coupling in an even mode is strong at the short-circuit end 1B. The odd mode coupling is capacitive coupling, and the even mode coupling is inductive coupling. The odd mode coupling and the even mode coupling cancel out each other. Accordingly, if the through holes 2a, 2b, and 2c have a cylindrical shape uniform in diameter, no coupling occurs between the inner conductors 3a, 3b, and 3c. Ordinarily, to obtain desired band-pass characteristics, the shape of the through holes 2a, 2b, and 2c and other factors are determined so that suitable coupling occurs between the inner conductors 3a, 3b, and 3c. In the case of the construction shown in FIGS. 3(a) to 3(c), the diameter of the through holes 2a, 2b, and 2c is reduced on the open end 1A side to weaken odd mode coupling while the diameter is increased on the short-circuit end 1B side to strengthen even mode coupling. Inductive couplings thereby occur between the inner conductors 3a, 3b, and 3c. The inductive couplings thereby provided attenuate signals having frequencies higher than the desired pass band. Also, the small-diameter portions of the inner conductors 3a, 3b, and 3c function as a small-capacity distribution constant line circuit.
In the conventional dielectric filter described above, the diameter of the inner conductors on the open end side is reduced in order to weaken capacitive coupling between the adjacent inner conductors. For this reason, the distance between the inner and outer conductor is large, and the areas of portions of these conductors facing each other are small, and the capacitances between the inner and outer conductors are small. For desired sharpness of the filter, skirt characteristics, however, a certain capacitance between inner and outer conductors is required. If a large capacitance between inner and outer conductors is required, it is necessary to increase the axial size of the inner conductors in order to achieve that capacitance. It is therefore difficult to reduce the overall size of the dielectric filter.